1. Field of the Invention
The present invention relates generally to the monitoring of the inspired gases of medical patients and more particularly to a humidity/moisture sensor attachable to a patient in such a manner as to be able to sense moisture in the patient's airway. Additionally, there may be a monitor for displaying measures of humidity in a patient. The invention also relates to a method of monitoring humidity or inspired gases, with the sensing device being used in conjunction with an in-dwelling artificial airway that allows voluntary and/or mechanical respiration to be accommodated.
2. Background and Prior Art
The upper respiratory system--nose, pharynx, larynx, and trachea--provides the human body with a conditioning means for inspired air. The inspired air is warmed, humidified, and filtered by these structures. The air becomes saturated with moisture that it picks up from the mucus secreted by the goblet cells of the mucus membranes, as it is pulled into the lungs. The warming of the air occurs by the heat exchange that takes place with the extensive capillary bed of the structures of the upper respiratory system.
When a person requires an artificial airway--endotracheal tube, tracheostomy tube, etc.--the functions of the upper respiratory system are bypassed. This means that no matter whether spontaneous breathing is occurring or mechanical ventilation is being provided, the temperature and moisture content of the inspired or delivered gas is not going to be physiologic without artificial conditioning. The technologies to warm and moisten the inspired gas--cascade humidifiers, bubblers, sonic, HME's (Heat and Moisture Exchangers) are well known in the art.
The problem with which clinicians are faced is how to determine and accurately regulate the amount of moisture or relative humidity that is being delivered to the air and therefore the patient with an artificial airway. The literature is full of studies and reports of improper inspired gas humidification. The following are some of the complications reported as a result of improper humidification:
Inadequate Humidification can result in:
Heat loss, Visidity of secretions, Increase in tracheal tube plugging, Atelectsis (mucus plugs), Epithelial damage, Increase in infection rates, Destruction of cilia, and Decrease in surfactant.
Excessive Humidification can result in:
Water intoxication, Atelectasis (water droplets), Inhibition of surfactant production, and Pyrexia (fever).
The current way that clinicians are tracking the amount of moisture being delivered to a patient is through observations of water level changes in a humidifier reservoir per the time interval and the gas flow being delivered. What they do not take into account is losses that occur through condensation in the breathing tubing and due to temperature fluctuations. When a heated wire system is used there is no condensation, but the danger with using a heated wire is delivering dry gas unknowingly, because of the inherent lack of condensate as a visual indicator that some moisture is being delivered (there is never condensate with this method so its presence or amount can not be used as an indicator).
Heat and moisture exchanger (HME's) devices are passive devices to conserve the patient's own heat and moisture from the expired breath. These devices vary in performance which is specified as the moisture per gas volume delivered at select flow rates as tested in the laboratory under controlled conditions. The literature reports that there is a higher incidence of airway plugging with HME,'s use versus heated humidification.
Complications from inappropriate humidification result in longer overall hospital stays, longer ventilator dependency, longer time in high cost intensive care units, longer utilization of delivery and monitoring devices, and more disposable products consumed. The current state of the art recognizes the importance of proper humidification of inspired gases.
The literature is full of studies that compare the humidifying characteristics of different commercially available moisture generating or retaining devices such as heated humidifiers, heat exchangers, and heat and moisture exchangers. Tamow-Mordi, in Evidence of Inadequate Humidification of Inspired Gas During Artificial Ventilation of Newborn Babies in the British Isles, The Lancet, Oct. 18, 1986, discloses the use of an electronic hygrometer incorporated into a side stream sampling chamber that requires a means to draw gases off of the breathing circuit into the sampling chamber that houses the humidity sensor. This requires the act of drawing of gases out of the breathing circuit in some manner, and removing the gas from the breathing circuit results in humidity changes and condensation which could lead to inaccurate results and thus inappropriate amounts of moisture being supplied to a patient. He discloses the use of a relatively expensive integral sensor/monitor system that gets exposed to patient contamination and would require resterilization prior to use on another patient.
Ballard, in Humidification for Ventilated Patients Intensive and Critical Care Nursing (1992) concludes her 1992 study of cascade water bath humidifiers with the following:
"In summary, humidification of gases for those patients receiving both short and long term ventilation is of paramount importance. It has been stated, `equipment to monitor humidity is not sufficiently sophisticated to allow accurate breath by breath measurements of humidity in the airways.` (Shelly et. al. 1988) The only guideline for manufacturers of heated humidifiers is the international standard which is felt to be an adequate, rather than optimal humidity." The prior art known to the inventor clearly provides the foundation: 1. that there exists a patient threat with delivering improperly humidified inspired gas and that this is clinically recognized as such; 2. that the state of the art in monitoring the moisture content of the inspired gas is cumbersome, labor-intensive, and inaccurate; 3. that the use of electronic hygrometers have been in side stream placement, expensive, totally reusable units which are exposed to patient contamination, and which have been used for studying humidification device performance.
Because the response time of currently available, cost-effective humidity sensors is less than that required to sense and have recorded or displayed breath by breath (inspiration and expiration) humidity changes, the invention may have an embodiment to monitor and trend the humidity of only the inspired gas, over time, and therefore over many breaths. A typical breath rate is 20 breaths per minute. Using a 1:1 ratio of inspired to expired breath duration, a sensor must respond in less than 1.5 seconds to change, however, a 1:2 ratio is not uncommon where the inspired breath occurs in 1.0 seconds or less. The response of currently available, suitable and cost effective, humidity sensors is approximately 5.0 seconds and thus is too slow to be able to monitor breath by breath. However, since the parameter desired to be measured by the present invention is a trend of the humidity of the inspired breaths, (and the fact that a single dry breath is not damaging to a patient, but dry breaths over time are), monitoring and trending the humidity provides the safeguard required. It is not necessary to be able to monitor and record every breath. Even if the sensor only monitors every third breath, a clinician can see on the monitor very quickly if an overly dry or overly humid trend is developing and can quickly act correctively accordingly (or an automated feedback loop mechanism can adjust humidity if there are, for example 5 readings below a determined threshold humidity level. This will provide medical personnel the ability to tailor the humidification means to the type of ventilatory mode and patient condition without over or under humidifying.
Also, the ability to monitor the humidity of the inspired air will reduce incidence of complications, and reduce the length of hospital stay by insuring appropriate humidification to the lungs. Energy will also be saved by way of reduced energy requirements per patient by reducing time on a ventilator, intensive care monitoring, intensive care testing, and the reduced disposable products used. The present invention solves the problems of not being able to measure, monitor, or deliver humidity of inspired or delivered air in patients having an artificial airway. The invention provides better patient care and comfort, reduced medical cost, convenience, and infection control with a disposable sensor, and reduces the energy impact of a patient with an artificial airway.
For the most part, presently, the appropriate humidity is not being delivered to patients having artificial airways. Currently there is no means or system to continually monitor and trend humidity at a patient's artificial airway. It would be advantageous to be able to measure, monitor and deliver appropriate humidity to air inspired by or delivered to a patient with an artificial airway.